1. Field of the Invention
The present invention relates to a reversible buck-boost chopper circuit and an inverter circuit with the same. More particularly, the present invention can be adopted for drive control of electric traction motors of a vehicle, each of which provides drive torque.
In hybrid vehicles or fuel cell electric vehicles, an increase of a power supply voltage, that is, a direct-current (DC) battery voltage is desirable to reduce copper loss and/or switching loss. So far, approximately 300 V has been an upper limit of the battery voltage in view of circumstances, such as higher battery costs.
In a hybrid vehicle or fuel cell electric vehicle, in order to use a DC power supply voltage higher than the battery voltage, a chopper type boost DC to DC converter, for example, is provided for boosting the DC power supply voltage. The boosted DC power supply voltage is converted into an alternating-cent (AC) voltage by an inverter composed of a three-phase PWM (Pulse Width Modulation) bridge circuit so that the AC voltage is applied to a high-voltage AC motor. The combination of the circuits of the chopper type boost DC to DC converter and the inverter, which is disclosed in U.S. Pat. No. 6,590,360 corresponding to Japanese Unexamined Patent Publication No. 2001-271729, has already been put to practical use. The combination of the circuits is referred to as “boosted inverter circuit” hereinafter.
The conversion efficiency of the boosted inverter circuit is a product of the conversion efficiency of the boost DC to DC converter and that of the inverter, resulting in greater power loss, such as switching loss, in the whole of the boosted inverter circuit to increase. The increase of the power loss may make it difficult to properly cool the boosted inverter circuit. In addition, the boosted inverter circuit requires both the boosted DC to DC converter and the inverter; this requirement may cause the boosted inverter circuit to increase in size and weight, deteriorating the installability of the boosted inverter circuit into a vehicle.
In order to solve the problems set forth above, it is possible to omit the boost DC to DC converter from the boosted inverter circuit to allow the inverter to directly convert the DC power supply (battery) voltage into a lower AC output voltage applied to an AC motor. This conventional configuration, however, increases output current; therefore, it may make conduction loss and switching loss increase. The increases of the conduction loss and the switching loss may make it difficult to properly cool the inverter circuit. In addition, the low output voltage makes field weakening difficult, which is necessary for controlling regenerating current when the AC motor is driven as a generator at a higher rotational speed.
Specifically, in the boosted inverter circuit technology for drive control of traction motors in a vehicle, it has been especially desirable to adopt a boost DC to DC converter with low losses; this boost DC to DC converter is capable of controlling power regenerated from a load to control breaking of a vehicle.